ABSTRACT Worldwide, millions of children are exposed to toxic metals, including lead (Pb), arsenic (As), cadmium (Cd), and manganese (Mn), with important consequences for their health and development. Research findings con- firm the association between exposure to single toxic metals and neurobehavioral deficits in children. However, metal exposures rarely occur in isolation. Nevertheless, few studies to date have examined interactions among toxic metals on cognitive deficits or behavioral problems, often with mixed results. There is also little under- standing of the long-term consequences of multiple metal exposures on child outcomes. Thus, scarcity of data and methodological limitations have kept this field from making sound recommendations for the protection of children and pediatric practice. The long-term goal of this research is to understand how low-level exposure to multiple toxic metals affects neurobehavioral outcomes across childhood and adolescence, identifying potential synergistic effects of metals on children's outcomes. The objective of the proposed study is to determine how children's exposure to multiple metals affects specific cognitive functions and behavior through middle child- hood. The proposed study is a longitudinal cohort study that will take place in Montevideo, Uruguay and builds on previous research in 1st grade children. The study focuses on As, Cd, Mn, Pb because these occur at bio- logically relevant levels in the environment and biological samples collected from the Uruguayan children. The central hypothesis is that within low range of exposures, higher concentrations of As, Cd, Mn and Pb will be individually associated with poorer cognition and behavior over time, and that co-exposure to these metals will produce additive or multiplicative negative effects on neurobehavioral outcomes. The specific aims of this study are to: 1) describe the trends and variability of the long-term exposure to As, Cd, Mn and Pb, 2) charac- terize the extent to which body burden of the metals is associated with deficits in specific cognitive functions, and 3) characterize the extent to which the metals are associated with reported and observed problem behav- iors. The study population will consist of ~760 children. The study has a staggered design, entailing: 1) the fol- low-up of children who had been evaluated as 1st-graders in 2 previous studies (years 2010-2013), and 2) the enrollment in 1st grade and follow-up of additional cohorts of children. Hair and blood samples will be collected yearly, and children's cognition and behavior will be evaluated in 1st, 3rd and 5th grade. This approach is innova- tive because, in contrast to previous studies, in employs a longitudinal approach to examine interactions among multiple metals that occur at low levels in the environment, and employs specific process-oriented cog- nitive outcomes as opposed to IQ, which is a summative measure. This research is significant in its contribu- tion to the communication of health risks associated with exposure to multiple toxic metals, and in its potential contribution to approaches that will prevent or alleviate the developmental consequences of exposure.